The focus of this application is to determine how the mammalian immune system generates a response that provides protection against intracellular pathogens that replicate inside of specialized organelles. To gain novel insight into this process, this project has focused on determining the host mechanisms that provide protection against infection by the intracellular pathogen Legionella pneumophila. Host pattern recognition system controlled by the adapter proteins MyD88, Rip2, NLRC4, and Naip5 have been shown to trigger innate immune responses to L. pneumophila. The overall goal of this project is to determine how signals generated from these immune surveillance systems are integrated into a functional response that prevents lethal infection by L. pneumophila. The aims of this project are to determine cell-specific responses in the lung by identifying host cells contain intracellular bacteria, host cells that have received L. pneumophila effector proteins translocated into the cytosol by the Dot/Icm system, and host cells that are producing proinflammatory signaling molecules in response to infection. The second aim is to determine the cell- specific roles for host pattern recognition systems in the pulmonary response to L. pneumophila using bone marrow chimeric mice and transgenic approaches. The third aim is to determine how spatial and temporal activation of the inflammasome restricts intracellular replication of L. pneumophila by investigating the mechanisms that underlie caspase-1 activation through NLRC4, Naip5 and ASC. These data will provide new details on how immune signals are received and propagated during infection by an intracellular pathogen.